Synthetic rutile containing more than 90% TiO.sub.2 and titania rich slag containing about 80-85% TiO.sub.2 are the major feed stocks for the production of TiO.sub.2 pigment or TiCl.sub.4 from which titanium metal is produced by reduction. TiO.sub.2 has got a number of very important applications as pigments as well as in the paper, plastics, rubber and textile industries apart from its use as welding electrode flux material.
The demand for TiO.sub.2 pigment is increasing rapidly and as natural rutile supply is limited, there is pressing demand for utilising the abundantly available ilmenite for producing synthetic rutile, which can subsequently be used for further processing. Production of alternate feed stocks namely, synthetic rutile or TiO.sub.2 slag from ilmenite has gained importance in this context, which may bridge the demand supply gap of TiO.sub.2 pigment.
A number of methods have been reported for the production of synthetic rutile from ilmenite out of which, only a very few are significant for industrial production. Many reviews on the beneficiation of ilmenite for synthetic rutile have been published. The important ones are by S. K. Jain et al (Ind. J. Technol., 15(9), 1977, 398-402), P. K. Jena et al (Ban. Met., 5, 1973, 107-117), J. A. Kahn (J. Metals, July 1984, 33-38), Moldran Iuliu et al (Chem. Abstr., 84 (1976), 78476 p), S. Yamada, (Ind. Miner., (London), 100 (1976), 33-40).
The various methods available for processing ilmenite for the preparation of synthetic rutile can be classified under different heads based on the physical and chemical principles employed for the processing.
In the fusion method, ilmenite is fused with various alkali metal compounds such as Na.sub.2 CO.sub.3, NaHSO.sub.4, Na.sub.2 SO.sub.4, and ZnS for converting it to a leachable product. After fusion ilmenite is leached with acids such that titanium and iron go into solution from where they are recovered (E. M. Khairy, M. K. Hussain and K. A. Baraway (NML Tech. J, 10 (4), 1968; K. V. V. Nair, Bull. Central Res. Inst. Uni. Travancore, 11 (1952), 106; A. K. Sharova and A. A. Fotiyev, Izev. Sibirskogo Otd. Akad. Nauk. SSSR, 4, (1959) 52; Y. Tokimoto and H. Hattori, J. Chem. Soc. Jpn. Ind. Chem. Soc. 58 (1955) 654; S. Prasad and J. B. P. Tripathi, Ind. J. Appl. Chem., 21 (1958) 162; A. S. Gaskin and A. E. Ringwood, Australian Patent 222517 (1959); G. Jabsen, Norwegian Patent 21693 (1910), Chem. Abstr. 6, (1912), 2153; L. E. Barton, U.S. Pat. No. 1,201,541 (1917), Chem. Abstr., 11 (1917), 279; Fr. Patent 483780 (1917), Chem. Abstr. 12 (1918) 1000; Br. Patent 106585 (1916), Chem. Abstr. 11 (1917), 2575; Norwegian Patent 29194 (1918), Chem. Abstr. 14 (1920), 1418; Belgian Patent 447709 (1942), Chem. Abstr. 41 (1947), 7064); H. H. Hoekje and R. A. Kearley, (German Patent 1058463 (1959), Chem. Abstr. 55, (1961), 5892); T. Shino, T. Tanaka, Y.Tanaka and Y. Takimoto (Jap. Patent 8771 (1950)); A. K. Sharova and A. A. Fotipev, (Chem. Abstr. 53 (1959), 26717.
The disadvantages of the process are that it generates large quantities of effluents and has got corrosion problems. Due to the high cost of alkali and other fusion materials the process is also uneconomical.
In the direct acid leaching route, ilmenite is leached with mineral acids such as hydrochloric acid and sulphuric acid for the production of synthetic rutile. On leaching ilmenite with sulphuric acid, both TiO.sub.2 and iron oxide go into the solution while in the case of HCl leaching, only iron is selectively removed from ilmenite (E. N. Kramer, U.S. Pat. No. 2,437,164 (1948), British Titan products Co. Ltd., Br. Patent 1085359 (1967); Ching Lung Lo and T. S. Mackey, Wah Chang Corp. U.S. Pat. No. 3,193,376 (1965); G. S. Davar, Ind. Patent 124558 (1969); Kenzo Ishihara, Outline of Ishihara Shngyo Kaisha Ltd. (1970); Columbia Southern Chemical Corp. Br. Patent 795164 (1958); N. N. Murach and L. G. Povedskaya, USSR Patent 116155 (1958); N. A. Aawal, M. Rehman, S. A. Tarafder and A. M. S. Huq, Chem. Abstr. 85, (1976), 146321 g).
In another industrially important process, ilmenite is subjected to partial reduction for converting the iron oxide to ferrous state, which is subsequently subjected to acid leaching for the production of synthetic rutile. There are a number of processes falling under this category which include: Murso (R. I. Jaffe and H. M. Burte, Titanium Sci. and Technol. Vol. 1, (Eds) R. I. Jaffe and H. M. Burte, Plenum Press, New York, London (1973); Burastero (J. Burastero, Chem. Abstr. 89, (1978), 26787z); Kurata (T. Kurata, Emi Satoshi, O. Kunihiko, T. Tstutomu and S. Isamu, Jpn. Kokai, 7693714 (1976). In certain cases a pre-oxidation of ilmenite prior to reduction was found to be helpful for reduction. M. G. Mu Ismail, J. Amarasekara and J. S. N. Kumarasinghe, Intl. J. Miner. Process. 10 (2), (1983), 161-164) report a similar process for the production of synthetic rutile from Sri Lankan ilmenites. Kerr Mc. Corp. has suggested a method where ilmenite is first subjected to oxidation followed by reduction and acid leaching (Rado Theodore A. C. Kerr. Mc. Corp. U.S. Pat. No. 4,199,552 (1980). In another process ilmenite was completely reduced to convert iron oxide to metallic iron, which was then removed by aqueous aeration rusting in presence of a catalyst (B. F. Bracanin, R. J. Clements and V. John, Proc. Austr. Inst. Min. Metall. 275 (1980), 33-42). Synthetic rutile with about 2-3% iron was prepared from Indian ilmenites by the reduction of ilmenite followed by acid leaching (Annie George, V. S. Kelukutty, L. G. Radhika, P. N. Mohan Das and P. K. Rohtagi, J. Mater. Sci., 19 (5), 1984, 1522). Metallic iron from reduced ilmenite was extracted by electrolytic dissolution to give synthetic rutile with high TiO.sub.2 content (Allan Benjamin Wilson, Ger. Offen. 255 7411 (1977); Mori Tado Oshi, Kato Akemi and Kawakami Naboru, Jpn. Kokai, 77 128817, (1977)). I. E. Grey, M. J. Hollit, A. Brian, B. O'Brien, Australian Patent 9346047, (1993); Australian Patent 649946 (1994); U.S. Pat. No. 5,427,749, (1995)) disclose a process for the reduction of ilmenite with coal followed by acid leaching before or after aeration rusting. Japanese Patent 58199720 (1983) granted to Ishihara, Sangyo Kaisha discloses a process wherein reduced ilmenite is subjected to aqueous oxidation followed by acid leaching at high temperature. Ilmenite was first reduced and was then subjected to oxidation in an aqueous solution in presence of ozone or hydrogen which was then leached with acids (H. Walter, European Patent 612654, (1994)). Pending Indian Patent Application No. 1262/Del/97 (inventors: P. N. Mohan Das, A. D. Damodaran, K. H. Bhat, S. Velusamy and S. Sasibhushanan) discloses a process for the production of high grade synthetic rutile from Indian ilmenite by employing solid state reduction, oxidation and acid leaching.
Direct chlorination of ilmenite at 850-950.degree. C. in presence of carbon to produce TiCl.sub.4 and iron chloride, which are then subsequently separated followed by oxidation of TiCl.sub.4 to yield TiO.sub.2 is also reported (D. Wendel. D. C. Jr. U.S. Pat. Nos. 4,332,615 (1982); 4,085,189 (1978); J. K. Heymer, G. Stephan and H. Werner, Ger. Offen., U.S. Pat. No. 3,203,482 (1983)).
Reduction smelting of ilmenite in electric arc furnace is another important process commercialised in different countries. This is applicable in countries where electricity is cheap and abundantly available. The slag produced can be used for further processing and pig iron would be a by-product (G. W. Elger, D. E. Kishy, S. C. Rhoads, US Bureau of Mines Investigation Report RI 8140, (1976), 31; D. J. Swinden and D. G. Jones, Trans. Inst. Min. Metall. Sec. C, 87, (1978), 83-87; A. J. Merchant and N. A. Warner, Trans. Inst. Min. Metall. C, 101, (1992), 177-182; R. H. Nafziger, Trans. Inst. Min. Metall. C 87 (1978), 120). A product containing high TiO.sub.2 was obtained from the slag by grinding, oxidation, reduction and acid leaching under pressure at high temperatures, (Borowiec Krzysztof, Grau Alfonso E, Gueguin Michel, Turgeon Jean-Franxois, U.S. Pat. No. 5,830,420 (1995)). Simultaneous reduction and melt separation of ilmenite using plasma containing hydrogen, methane or argon are also reported for the production of titania rich slag and pig iron (J. D. Chase and J. F. Skirvan, AIChE Symp. Ser. 75, 186, 38(1978); R. Izhizuka and K. Akashi, J. Jap. Inst. Metals, 45, 1229 (1981); P. K. Mishra, S. K. Singh, D. C. Acharya, D. C. Mohanty and P. K. Sahoo, Min. processing, Recent Advances and Future Trends, Conf. proc. (1995), 875-878, (Eds), S. P. Mehrotra and Shekar Rajeev, Allied Publ. New Delhi; G. M. Denton, A. Schoukens and S. Francois, European Patent 583126 (1994). Warner has reported another method for the production of synthetic rutile (Warner Noel Alfred, Br. Patent 9211052, 92110523, (1992)). A two-stage process involving pelletisation of ilmenite and its pre-reduction followed by submerged arc smelting is reportedly in use in Norway at Tinfos Titan & Iron KS (Metal Bulletin Monthly, September 1996, 67-71). QIT, Canada has developed a process for upgrading their Sorel slag to result in a new proprietary product containing 95% TIO.sub.2 and acceptable to chloride-route for pigment manufacturing (Metal Bulletin Monthly, September 1996, 67-71).
Reduction smelting of ilmenite for slag and pig iron is highly energy intensive and as such the process is quite uneconomical in countries where electrical energy is scarce and expensive. The slag thus produced may require further processing for use as a starting material for chlorination. The wet processes used commercially have the disadvantages of generating large quantities of effluents. The reduction smelting processes using electric arc or plasma are more environmentally friendly as they produce less pollution and result in a saleable byproduct in the form of pig iron.
Electric arc smelting of ilmenite is highly energy intensive and requires more time for smelting thus making its throughput very low compared to plasma smelting. Indian Patent Application No. 804/Del/97 of A. D. Damodaran, P. N. Mohan Das, K. H. Bhat, B. C. Mohanty and P. S. Mukherjee, discloses a process involving the smelting of pre-reduced ilmenite in plasma for the production of TiO.sub.2 slag and pig iron. The pre-reduced ilmenite in the smelting charge reduces the energy requirement while the plasma renders the process more clean and productive with high throughputs and minimum energy loss. The slag produced by this method had about 80-85% TiO.sub.2 content while the pig iron byproduct had a metallisation of 99.76%.